To get in shape for her October wedding, Dawn-Samantha Cahill, 25, a production coordinator in New York City, tried every exercise routine she could think of. But the upended positions that yoga required made her feel self-conscious. Running on a treadmill was a bore. And lifting weights was just too difficult. Frustrated, Cahill started overeating instead of exercising, and her weight, on a petite, 5-ft. 2-in. frame, ballooned to 212 lb.

Cahill was nearly ready to give up, until one day last spring, she discovered a video on YouTube about hula-hooping. "The woman in the video was...